Turn on cast shadows for cave mesh. Also this is very unoptimal to create huge static meshes like this. Everything will be fine as long as you do not use dynamic light, but if you do more then 4 in this level FPS will tank.
Second problem i see here is collision, you may get collision glitches because that cave is single (inverted) surface.
So you have choice to make set of small meshes, then world coordinate material (to make texture seamless). Or battle all those shadows, dynamic lights and weird collision bugs later. Just a warning.
Ps watch videos about creating “the solus project” there are lots of good tips about making cool caves.
PPS. you also can split this huge mesh into sections, add UCX collision meshes, and make it cast shadows. But imo its more work and worse effect compared to solus workflow.